1. Field of the Invention
The present invention relates to a negative electrode active material for a secondary battery and a method for producing the same.
2. Description of the Related Art
Secondary batteries have been widely used as electrical power sources for driving various electrical devices. As is well known, a secondary battery can be repeatedly charged and discharged, and thus can be used repeatedly. For example, in a lithium ion secondary battery, materials capable of releasing and storing lithium ions are used in positive and negative electrodes, and the lithium ions are transferred between the electrodes in charging and discharging cycles. In charging and discharging reactions, the active materials in the positive and negative electrodes act to release lithium ions to the outside and to absorb lithium ions from the outside, and thereby undergo changes in volume.
For example, aluminum, which has a lithium ion storage capacity higher than that of carbon materials or the like, has been proposed for use as the negative electrode active material for a secondary battery. However, the volume of a negative electrode active material that is composed of aluminum is likely to be changed excessively during charging and discharging reactions. Therefore, this material often becomes deteriorated (e.g., converted into a fine powder or dropped off) in reactions, which tends to shorten the life cycle of the secondary battery.
For example, Japanese Laid-Open Patent Publication No. 63-318070 and Japanese Laid-Open Patent Publication No. 03-152874 propose an aluminum alloy, which is usable as a secondary battery negative electrode active material with high capacity and improved durability. More specifically, as disclosed in Japanese Laid-Open Patent Publication No. 63-318070, 20 wt % or less of magnesium is added to an aluminum melt to prepare an aluminum-magnesium alloy as the lithium secondary battery negative electrode active material. On the other hand, in a non-aqueous electrolyte battery, as described in Japanese Laid-Open Patent Publication No. 03-152874, 15.0 wt % or less of magnesium is added to aluminum to prepare a solid solution of an aluminum alloy as the negative electrode active material.